RC fuel systems

Model Airplane News, Aug 2002

Tanks, tubing & fittings-- making it all work!

Flying RC model airplanes that are powered by glow and gasoline engines means that you'll have to Install a fuel tank and other necessary fuel-system components. This is easy for experienced builders, but newcomers often find it challenging to choose, assemble and Install a proper fuel-delivery system. This article shows the basic makeup of typical fuel systems and how to properly Install and maintain them. Whether you want to pilot 1/2A sport models, speedy pylon racers, or giant-scale warbirds, you have to keep the fuel flowing to the engine. These tips will make all that plumbing more understandable.

THE BASICS

The fuel tank is a container for the engine's supply of fuel. The tank is connected to the engine's carburetor with flexible fuel line (often called fuel tubing), and a rubber stopper seals it. For a tank to operate properly, it must have a vent line that allows air to enter the tank as fuel is drawn out. The vent relieves the vacuum left in the tank. Model airplanes don't always fly straight and level; they climb and dive and often fly inverted. To allow the fuel to flow at different attitudes, the tank has a flexible internal pick-up tube. A heavy fitting (called a "clunk") at the end of the pick-up tube keeps the end of the tube at the lowest part of the tank for a continuous supply of fuel. Lengths of brass tube pass through the tank's stopper, and the flexible fuel lines that carry the fuel to the engine simply slip over the brass tubes. The rest of the fittings and accessories help the fuel system work properly and make it easier to maintain and operate.

Once you've chosen the correct tank, fuel lines and accessories for your model, you'll be well on your way to having an engine that runs reliably.

HOW MANY LINES?

* TWO-LINE SETUP. A two-line fuel system is very simple and almost foolproof. To set up the tank, you need only two pieces of brass tube, a clunk, a rubber stopper and a short length of silicone tubing. Bend one tube 90 degrees to form the vent and insert it through the stopper. It lets outside air in as the fuel is drained out, and it acts as an overflow indicator when you fill the tank. The second tube is the fuel-supply tube; the pick-up tube and clunk are attached to it. This line is attached to the carburetor and is also used to fill the tank.

* THREE-LINE SETUP. In a three-line tank, you assemble the tank just as you do a two-line system, but the third line is used to fill the tank. It can be made from a short length of brass tube and doesn't need a pick-up line in the tank; you can also make it like the vent tube, except it is pointed downward. Before you start the engine, you must plug the third line to prevent fuel from leaking out.

GASOLINE FUEL SYSTEMS

Perhaps the simplest fuel systems are those for gasoline-powered engines. Unlike glow engines, which use a venturi vacuum to draw the fuel into the carb, most gas engines have a pumper carb to draw fuel out of the tank. For this reason, the placement of the gas tank is not nearly as critical as it is for the glow engine. A gas engine can draw the fuel through several inches of tubing without any ill effects. Also, because the gasoline carb has a diaphragm pump, it won't allow fuel to flow into the engine unless it is running.

An easy way to fill and empty the tank is to install a T-fitting in the supply line and add a length of tubing. B When you pump fuel into this line, it will flow only into the tank and won't flood

the engine. When the tank is full, you can use a plug to seal the fill line, and the engine will draw the fuel from the tank. By adding a long fill line and using a fuel dot fitting, you can place your filler cap anywhere you like-great for scale models.

Properly installed, your fuel system will last a very long time and may never need to be changed. In a hard landing, however, some of its parts may be dislodged and stop working. Here are some common fuel-flow problems and fixes.

* If your engine begins to run lean, check for small pinholes in the fuel-supply line. Check closely wherever there are tight bends in the line, or where it comes into contact with your model. Leaks commonly occur where the lines pass through the firewall. A better method of installation is to drill small holes in the firewall and use lengths of brass tubes in the holes. You can then slip the fuel lines over the brass tubes to complete the system (see figure 6).

* After a hard landing, the flexible pick-up tube and dunk inside the fuel tank may be forced all the way forward. This often goes unnoticed until the next flight when the tank stops delivering fuel to the engine in a nose-high attitude. To prevent this, solder a short piece of brass tube to your dunk This decreases the pick-up tube's flexibility but still allows it to draw fuel in normal flying attitudes (see figure 7).

* If your engine begins to run erratically, chances are that some debris has gotten into the fuel system and is blocking the carb. It usually finds its way into the fuel tank from your fuel jug, and if it blocks the fuel flow, your engine will die. The easiest way to prevent this is with an in-line fuel filter. You install it just before the car) in the supply line. You can also install a filter in your fuel-pump line so you fill the tank only with filtered fuel. Add a combination fuel clunk/fitter, and you'll have a triple defense against deadsticks.